This invention relates to superconducting magnet windings in general and more particularly to an improved method for constructing a superconducting magnet winding.
Superconducting magnet windings which contain several winding layers arranged parallel to each other, between each of which a separator of insulating material, forming cooling ducts, is inserted, and which is surrounded, at least over parts of its outer surface, by a hardenable material which is worked down to a predetermined dimension after the hardening process are known.
Magnet windings with superconductors can advantageously be used for producing magnetic fields of large volume. If the superconductors of these windings are cooled down to a temperature below the so-called transition temperature of the superconductive material used for the winding by means of a coolant, generally by means of liquid helium, the ohmic resistance of the superconductive material disappears almost completely. Because of the correspondingly reduced power required, superconducting magnets therefore offer the advantage, over conventional magnets with windings of electrically normally conducting material such as copper, that stronger magnetic fields and thus, also greater magnetic field gradients can be obtained thereby. Such magnet windings are needed, for instance, for fusion reactors, the strong magnetic fields of which are used to confine a hot plasma by means of magnetic forces and thereby to make a fusion process in the plasma possible.
A further field of application of such superconducting magnet windings are as support or lateral guidance magnets for a magnetic suspension system which allows contactless guidance of a vehicle along a stationary track according to the electrodynamic repulsion principle.
In addition, suitable superconducting windings can also be provided for the deflection or focusing of a beam of charged particles for instance, in particle accelerators.
In order to obtain very strong magnetic fields or large magnetic field gradients, the effective current densities in their superconducting conductors must generally be chosen very high. This may make loading of the superconductors up to near their critical current necessary. Such conductors must be protected, particularly from mechanical instabilities which can be caused by conductor movements. For, if a superconductor with a magnet winding has the possibility of moving under the action of an external force, for instance, due to a variable magnetic field, then it can heat up, due to the friction heat connected with such movement or due to the kinetic energy converted into heat, to such an extent that its transition temperature is exceeded and it becomes normally conducting, at least at the location of the mechanical instability.
In order to prevent such instabilities of the mechanical kind and the warming up of the conductors connected therewith, the individual layers of the winding of a superconducting magnet winding can be impregnated, in a known manner, with a material which is hardened and thereby bonds the winding layers firmly to each other. It must be ensured, however, that the superconductors of the winding are sufficiently well cooled by a cryogenic medium. The cooling ducts required therefor can be obtained, for instance, by installing separate inserts when the winding is made. These inserts correspond to the cooling ducts in the magnet winding; they can be removed from the winding after the impregnating process is completed, i.e., after the impregnating material has set British Pat. No. 1,443,207.
Since, in larger superconducting magnet windings, large current densities provided, correspondingly large forces which bulge out the winding also occur. These windings must frequently be exposed to a large mechanical pretension at their outer circumference. This is generally accomplished by means of special components such as wedges, screws or cup springs, which are fitted between an armor and the outer cylindrical surface of the winding. With these measures, unpermissible movements of the superconductors and, if necessary, also movements of the entire windings within a winding housing, which can likewise make the winding normally conducting, are prevented. A corresponding fixation of a winding in a winding housing is known, for instance, from the German Offenlegungsschrift No. 24 59 104, pages 4 to 6 and FIGS. 1 to 5.
With this known method of fabrication, the magnet winding, which contains several winding layers which are arranged parallel to each other and between each of which a separator is inserted, is first surrounded, at its outer, relatively irregularly shaped sidewall surface, with a hardenable material. After hardening, the material is then worked down to a predetermined dimension. Thereupon, the separators between adjacent winding layers, are replaced by corresponding insulating layers which contain cavities for conducting a coolant, and the individual parts of the winding are cemented together. The winding assembled in this manner can then be inserted into a housing and can be prestressed, at its outer surface which was worked down to the predetermined shape, using suitable intermediate elements such as wedges. This known method for constructing a winding, however, is relatively laborious.